A whistle stop tour of changing sea level

First,  before I start looking at the hows, and the whys, and the whodunnits I want to put everything into context with a brief overview of how global mean sea level has changed since the oceans first formed ~4 billion years ago (Nutman, 2006).

Hallam (1984) reconstructed sea level from the Neoproterozoic using sequence stratigraphy and marine epicontinental cover. It is worth bearing in mind that with increasing age, and the inevitable erosion of data, the error margins are larger, but nevertheless it does show something interesting: global mean sea level has varied hugely over the last 590 million years (Fig. 1). Crucially the reconstruction suggests that sea level has been higher than today for most of geological history, only coming close to present day levels around the Permian - Triassic boundary and then more recently during the Cenozoic.



Figure 1: Sea level change for the last 590 million years as inferred from seismic sequence stratigraphy. Graph adapted from  Lambeck and Chappell (2001) using Hallam's data.

So what’s all the fuss about? Fluctuating sea level is not a new thing; in fact, it’s pretty much routine, happened loads of times before, maybe this subject is getting a little old? Maybe I should change my blog topic...

 

But let's zoom in on more recent history (still geological history mind!). Fleming et al (1998) reconstructed global mean sea level (GMSL) since the Last Glacial Maximum (LGM) using data from thirteen locations with an emphasis on farfield sites (Fig. 2). Farfield sites are important because they were furthest from the ice sheet coverage during the LGM so are not affected by isostatic rebound. Because of this reconstructions based on data from these sites are thought give a better indication of GMSL during the Holocene (Milne et al, 2005). Figure 2 shows sea level rising by ~120 metres after the last ice age and this appears to be followed by a period of relatively stability from 7ka to the present day. 

So there is relatively constant sea level for thousands of years as our ancestors are populating the world, and choosing where to bring up baby Ugg. And if the sea level's not going to change why wouldn't you build your hut with a sea view?
 

 

Figure 2: Sea level reconstructions for (a) the period following the LGM,and (b) for the Holocene (Fleming et al, 1998).

Zoom in again, and this time with the most accurate data yet. Sea level has been measured using tide gauges from around the world, and since 1993, satellite altimetry can be used too. Figure 3 compares several different reconstructions of eustatic sea level from tide gauge data since 1880. While there are differences in the individual reconstructions, all of them show that the period of relative stable sea level is over and that sea level is rising.



Figure 3: Reconstructed eustatic sea level using various methods and tide gauge data (IPCC)

 

On top of this rising sea level the IPCC has stated that:

'It is very likely that there will be a significant increase in the occurrence of future sea level extremes in some regions by 2100, with a likely increase in the early 21st century'

A slight problem then for the millions of us who are living near the coast, or in some cases already living below sea level.

Finally, I want to leave you with one more image. Figure 4 shows the global changes in sea level as measured by satellite altimetry from 1993 - 2008. Clearly, the rate of sea level change is variable across the globe. I'm excited to find out how the countries in a whole new kind of red are coping with the sea level rises and what plans (if any) they have for the future.



Figure 4: Sea level changes from 1993 - 2008 as measured by satellite altimetry from the Laboratory for Satellite Altimetry